Recently, a ring oscillator in which a single end is adopted is mounted in a SOC (System-On-a-Chip) system. Ordinarily, the ring oscillator includes a transistor such as a CMOS (Complementary-Metal-Oxide-Semiconductor) transistor and a bipolar transistor, odd-number inverters cascade-connected, and a current source or a voltage source including a transistor and a battery.
However, a noise component such as a flicker noise and a thermal noise is mixed in a signal generated by the current source or the voltage source of the ring oscillator. The flicker noise caused by the transistor tends to emerge in a low frequency band and is inversely proportional to a frequency. The thermal noise is caused by random thermal motions of carriers (electrons) and independent of the frequency. The noise component adversely affects the frequency of the signal oscillated by the single-end oscillator included in the ring structure.
On the other hand, a differential ring oscillator that oscillates a signal corresponding to a difference between two signal components is well known as a device to reduce the noise component. The differential ring oscillator includes two output terminals. There is a phase difference n between the two signals output from the tow output terminals. In other words, in the differential ring oscillator, a potential at one of the output terminals is lowered when a potential at the other output terminal is raised.
However, in the differential ring oscillator, ordinarily, it is necessary to avoid in-phase oscillation and sticking of in-phase potential. The in-phase oscillation means that a phase difference between the two output signals becomes zero. The sticking of in-phase potential means that each of the two output signals freezes at a stabilization point such as a power supply or a ground. In order to avoid the in-phase oscillation and the sticking of in-phase potential, the conventional differential ring oscillator includes a complicated circuit that prevents the oscillation of the output signal. Therefore, a circuit size and a consumption current are increased, and circuit design is difficult to establish.
That is, in the conventional differential ring oscillator, the noise component of the output signal is hardly reduced without increasing the circuit size and the consumption current (see JP-A No. 2006-339871 (Kokai) and “2 GHz CMOS Noise Cancellation VCO” written by Amit Bansal and Chun-Huat Heng, and IEEE Asian Solid-State Circuits Conference Nov. 3-5, 2008, page 461 to 464). Further, the noise component of the output signal is hardly reduced using a simple circuit whose circuit design is easy to establish. In other words, there is a trade-off relationship between the reduction of the noise component and the improvements in circuit performance (e.g. the reduction of the circuit size and the consumption current, and the simplification of the circuit design).